Nano tungsten trioxide for ceramic microsphere may be synthesized by hydrothermal method. If the pH value of the reaction system is 1.0 and 1.5, the synthesized product is tungsten trioxide of hexagonal phase. And if the pH value is 2.0 and 2.5, the synthesized product is tungsten trioxide of cubic phase.
According to the analysis, the reason why the synthesized tungsten trioxide is different in phases is that the H+ concentration of the reaction system is different during the adjustment of the pH value in the reaction system using HCl and different tungstic acid/polytungsten acid sols are generated. In the low-temperature hydrothermal conditions, the W/O re-bonds in sol, and then crystallizes to generate metastable tungsten trioxide, and H+ can be separated. Therefore, in the process of dynamic equilibrium reaction, after appropriately increasing the H+ concentration of the system, the hydrothermal crystallization of tungsten trioxide can be suppressed to a certain extent, so that the grain size of tungsten trioxide is reduced. If the H+ concentration is further increased and the powder further crystallized, it may lead to the distortion and tilt of tungsten octahedron and the displacement of the tungsten atoms from the octahedral center to the octahedral edge. Thus, tungsten trioxide converts from the hexagonal phase to cubic phase.
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